Crankcase lubricants for passenger car and heavy duty diesel engines contain numerous additives providing the lubricant with an array of performance properties required for optimum function and protection of the respective engines. Each individual additive is requires to provide the performance benefit for which it was designed without interfering with the function of the other additives in the lubricant. Within each additive class (e.g. dispersant or detergent) a number of options are available that differ in structure, such as molecular weight, metal type, hydrophobic/hydrophilic balance, etc. The selection of the additives for any given formulation must take into account both the relative performance characteristics of the individual additives, as well as synergies or antagonisms with other additives present in the oil.
Additive packages containing multiple additives are typically sold to lubricant formulators in the form of concentrates, to enable the introduction of a range of base stocks to target different viscosity grades, performance levels and costs. This leads to further complications in that the selected additives must be compatible with each other in the concentrate to avoid additive package instability and phase separation.
In some cases, the most desirable additive structure from a performance standpoint interacts more strongly in the concentrate compared to other alternatives. The use of a combination of overbased colloidal sulfonate and hydroxybenzoate (such as salicylate) detergents is an example. A combination of overbased colloidal sulfonate and hydroxybenzoate detergents, together with high molecular weight succinimide dispersants, has been found to provide optimal cleanliness and acid neutralization efficiency, together with high molecular weight succinimide dispersants for sooted oil rheology control in crankcase lubricating oil compositions for heavy duty diesel (HDD) engines. These additives, however, exhibit incompatibilities that limit the combined use thereof in the form of an additive concentrate. Surprisingly, it has now been found that, while the combination of a high molecular weight succinimide dispersant and conventional overbased colloidal hydroxybenzoate and sulfonate detergents result in an additive concentrate results in concentrate stability issues, high molecular weight succinimide dispersant and an overbased colloidal hybrid detergent derived from a mixture of hydroxybenzoate and sulfonate surfactants are compatible and that additive concentrates containing such dispersants and detergents remain stable over a range of compositions.